Americans, it seems, are rediscovering the pleasures of their own backyards. For many what was once a place for casual barbecues with friends has now become a full-fledged room with all the amenities of the indoors: gourmet appliances, light fixtures, cushioned furniture and even flat-screen televisions. In colder parts of the country, homeowners are putting in heat lamps and fireplaces so they can lounge outside almost any time of year.